how to interpret the camera world matrix?
The solution is so obvious, I must be getting old :( Anyway, the world matrix of the camera is a transformation matrix, not a viewing matrix. To obtain a viewing matrix out of it, you need to invert (transpose) it and zero out the translation part.

Explanation:

We set all Euler angles of the camera to 0, so the local axes are aligned with the world axes (the "identity", non-transformed camera):

We see, that this camera is viewing in the negative local z direction, if we rotate by 90 degrees around the x axis we get the picture in the question. The columns of the world matrix being the transformed axes of the "identity" camera:

x column: [1,  0, 0]
y column: [0,  0, 1]
z column: [0, -1, 0]


To obtain a viewing matrix of the camera (for OpenGL, say), we need to transpose (write the columns as rows) and set the translation part to 0:

[1,  0, 0,  0]
|0,  0, 1,  0|
|0, -1, 0,  0|
[0,  0, 0,  1]


we can see the projection plane is formed by the world x- and z-axes, the local z-axis of the camera is aligned with the world -y-axis, but we view along the negative camera z direction, hence the world y-axis and so can see the cube.